Predators & Defenses
  black dot


  Defenses relating to spines are considered here, while HIDING/SHELTERING/COVERINGS and PEDICELLARIAE are considered in other sections.
  black dot


  black dot
Research study 1

photograph of a dried test of a sea urchin Strongylocentrotus franciscanusSpines are the first line of physical defense for a sea urchin.  The spines are constructed of calcium carbonate secreted over a scaffolding of protein.  Upward-facing barbs on the spine add to tearing of the flesh of a predator on penetration.  There are no poison glands in the spines but, based upon the degree of pain inflicted on a punctured human, the skin covering the spine must have some toxic properties.  The spines are sited, ball-and-socket fashion, on bumps on the test and, through the action of muscles attached around the circumference of the spine, can be moved through any angle in any direction.

Aggregations of red urchins Strongylocentrotus franciscanus may be so dense as to
trap sea stars, such as this sun star Solaster stimpsoni, within a vast canopy of spines

photograph of several red urchins Strongylocentrotus franciscanus with a sea star Solaster stimpsoni
  black dot

Apart from defense, what other functions might the spines have? Consider the answers given, then CLICK HERE for explanations.

Obtaining food. 

Exchanging gases for respiration. 

Wedging into crevices. 


Competing with other sea urchins. 

Interacting sexually. 

  black dot
photograph of a red sea urchin Stronglylocentrotus walking on its spines taken from a video

CLICK HERE to see a video of a red sea-urchin walking on its spines when emersed. This demonstration, which leads to no harm to the sea urchin, emphasises how strong and sturdy
are the spines.

NOTE the video replays automatically

  black dot
Research study 2

Laboratory studies on sea urchins and spiny lobsters Panulirus interruptus at Santa Cruz Island and San Diego, California show that the lobsters clearly prefer purple urchins Strongylocentrotus purpuratus over red urchins S. franciscanus of the same test diameter.  graph showing comparative test thicknesses of sea urchins Strongylocentrotus franciscanus and S. purpuratusThey also prefer small red urchins over large ones.  Small urchins (<30mm test diameter) of both species are entirely consumed.  As urchins get larger, the proportion of test consumed progressively gets smaller, and is zero for large red urchins (>90mm).  Many of the smallest red urchins (<30mm) appear to gain protection from the spine canopies of adults, but mid-sized individuals too large to fit under the canopy are heavily preyed upon. 

size-frequency distributions of sea urchins Strongylocentrotus franciscanus in Santa Cruz Island, California This may explain the comparative absence of mid-sized individuals in field collections of S. franciscanus at San Clemente Island (see graph on Left). Comparisons of test and spine morphometries of the two sea-urchin species show that while test thickness is slightly (0.2mm), but significantly, thicker in purple urchins (figure at Right), spine length in red urchins is considerably greater (figure below Right).

graphs showing comparative spine lengths in sea urchins Strongylocentrotus franciscanus and S. purpuratusThe authors suggest that the difficulty involved in handling large-spined red urchins may explain the lobsters’ preference for the shorter-spined S. purpuratus.  The authors add that heavy fishing on lobsters in the area may have increased sea-urchin population numbers and contributed to episodes of destructive sea-urchin grazing in the area during the 1950s and later.  Tegner & Levin J Exp Mar Biol Ecol 1983 73: 125.



black dot
Research study 3


photograph of purple sea urchins Strongylocentrotus purpuratus in a tidepoolSpines in sea urchins are useful in defense but, if too long, are susceptible to being snapped off in high wave areas by moving seaweeds and water-borne debris.  Studies at Hopkins Marine Station, Pacific Grove, California show also that longer spines increase lift and drag forces in moving water.  Of the 3 predominant west-coast Strongylocentrotus species., spine length in purple urchins S. purpuratus seems to be the best example of evolutionary compromise. The spines are long enough to provide at least some defense from predators, yet short enough to enable shell and rock bits to be held tight to the spines for protection.  This last behaviour may enable the species to live intertidally in areas of high wave-energy.  In comparison, with their relatively longer spines, red and green sea urchins are generally limited to subtidal and/or protected low intertidal regions.  Denny & Gaylord 1996 J Exp Biol 199: 717.




Large bed of purple urchins S. purpuratus in protective hollows
in a high wave-energy location at Botanical Beach, British Columbia

  black dot